The present invention relates generally to recovery of symbol timing and sub-carrier frequency information from orthogonal frequency division multiplexed (OFDM) digital communications signals. More specifically, but without limitation thereto, the present invention relates to a circuit for synchronizing OFDM signals subjected to multi-path fading and additive noise.
OFDM signals comprise several concurrently transmitted and independently modulated sub-carrier signals. The sub-carriers are spaced in frequency at the reciprocal of the OFDM symbol period such that the sub-carriers are all mutually orthogonal in the ordinary mathematical sense. OFDM signals typically contain several hundred or more sub-carriers, and the bandwidth occupied by an OFDM signal equals the number of sub-carriers times the symbol rate. Because each sub-carrier is independently modulated, the temporal and spatial characteristics of OFDM symbols are similar to those of broadband noise.
Synchronization of the sub-carrier frequencies and the symbol timing is desirable to extract and decode information from the OFDM signal, particularly in wireless communications systems where multi-path channel effects introduce more distortion and frequency offset than wire line or cable communications systems. A need therefore exists for an OFDM frequency/timing recovery circuit that can generate accurate symbol timing and sub-carrier frequency information even in the presence of severe frequency selective multi-path fading and additive noise.
The present invention is directed to overcoming the problems described above, and may provide further related advantages. No embodiment of the present invention described herein shall preclude other embodiments or advantages that may exist or become obvious to those skilled in the art.
An OFDM frequency/timing recovery circuit of the present invention provides accurate recovery of both sub-carrier frequency and symbol timing information from a single OFDM symbol. Accuracy may be further enhanced in demanding applications by utilizing two OFDM symbols. A unique frequency/timing recovery symbol is used that is an OFDM symbol in which each sub-carrier is differentially phase modulated with respect to each adjacent sub-carrier with random information. The phase modulation values contained in the frequency/timing recovery symbol are fixed and known by the receiver. Also, they are selected in such a way that they contain two or more identical fractional parts, for example, identical first and second halves of the frequency/timing recovery symbol. A correlator is used to compare redundant portions the input RF/IF signal symbols until a correlation peak magnitude is found. The sample time at the correlation peak provides an estimate of symbol timing. The phase of the correlation peak provides an estimate of carrier frequency offset modulo the sub-carrier spacing. Using these initial estimates, the frequency/timing recovery symbol is processed a second time to refine the estimates. The differential phase modulation is recovered from the frequency/timing recovery symbol and correlated with the known phase modulation values. The location of the peak of the correlation function yields the integer number of sub-carrier frequency offsets and the phase yields a refined estimate of the symbol timing.
An advantage of the frequency/timing recovery circuit of the present invention is the amount of time, measured in numbers of transmission symbols, required to achieve synchronization.
Another advantage is that most of the symbols in a information packet may be dedicated to information rather than synchronization.
The features and advantages summarized above in addition to other aspects of the present invention will become more apparent from the description, presented in conjunction with the following drawings.